The presence of large deposits of oil shale in the high plateau, semi-arid region of the western United States has given rise to extensive efforts to develop methods for recovering shale oil from kerogen in the oil shale deposits. It should be noted that the term "oil shale" as used in the industry is, in fact, a misnomer; it is neither shale nor does it contain oil. It is a sedimentary formation comprising marlstone deposit with layers containing an organic polymer called "kerogen" which, upon heating, decomposes to produce liquid and gaseous products. It is the formation containing kerogen that is called "oil shale" herein and the liquid hydrocarbon product is called "shale oil".
A number of methods have been proposed for processing oil shale which involve either first mining the kerogen-bearing shale and processing the shale on the ground surface, or processing the shale in situ. The latter approach is preferable from the standpoint of environmental impact, since the treated shale remains in place, reducing the chance of surface contamination and the requirement for disposal of solid wastes.
The recovery of liquid and gaseous products from oil shale deposits has been described in several patents such as U.S. Pat. Nos. 3,661,423; 4,043,597; 4,043,598; and 4,192,554; and in U.S. patent application Ser. No. 070,319 filed Aug. 27, 1979, by Chang Yul Cha, entitled TWO-LEVEL, HORIZONTAL FREE FACE MINING SYSTEM FOR IN SITU OIL SHALE RETORTS and now abandoned. Each of these applications and patents is assigned to Occidental Oil Shale, Inc., assignee of this application, and each is incorporated herein by this reference.
These patents and applications describe in situ recovery of liquid and gaseous hydrocarbon materials from a subterranean formation containing oil shale, wherein such formation is explosively expanded to form a stationary fragmented permeable mass of formation particles containing oil shale within the formation, referred to herein as an in situ oil shale retort, or merely as a retort. Retorting gases are passed through the fragmented mass to convert kerogen contained in the oil shale to liquid and gaseous products, thereby producing retorted oil shale. One method of supplying hot retorting gases used for converting kerogen contained in the oil shale as described in U.S. Pat. No. 3,661,423, includes establishing a combustion zone in the retort and introducing an oxygen-supplying retort inlet mixture into the retort to advance the combustion zone through the fragmented mass. In the combustion zone, oxygen from the retort inlet mixture is depleted by reaction with hot carbonaceous materials to produce heat, combustion gas, and combusted oil shale. By the continued introduction of the retort inlet mixture into the fragmented mass, the combustion zone is advanced through the fragmented mass in the retort.
The combustion gas and the portion of the retort inlet mixture that does not take part in the combustion process pass through the fragmented mass on the advancing side of the combustion zone to heat the oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition, called "retorting". Such decomposition in the oil shale produces gaseous and liquid products, including gaseous and liquid hydrocarbons, and a residual carbonaceous material.
The liquid products and the gaseous products are cooled by the cooler oil shale fragments in the retort on the advancing side of the retorting zone. The liquid hydrocarbon products, together with water produced in or added to the retort, collect at the bottom of the retort and are withdrawn. An off gas is also withdrawn from the bottom of the retort. Such off gas can include carbon dioxide generated in the combustion zone, gaseous products produced in the retorting zone, carbon dioxide from carbonate decomposition, and any gaseous retort inlet mixture that does not take part in the combustion process.
U.S. Pat. Nos. 4,043,597; 4,043,598; and 4,192,554 disclose methods for explosively expanding formation containing oil shale toward horizontal free faces to form a fragmented mass in an in situ oil shale retort. According to such a method, a plurality of vertically spaced apart voids of similar horizontal cross-section are initially excavated one above another within the retort site. A plurality of vertically spaced apart zones of unfragmented formation are temporarily left between the voids. A plurality of horizontally spaced apart vertical columnar explosive charges, i.e., an array of explosive charges, is placed in each of the unfragmented zones and detonated to explosively expand each unfragmented zone upwardly and/or downwardly towards the void or voids above and/or below it to form a fragmented mass having an average void volume about equal to the void volume of the initial voids. Retorting of the fragmented mass is then carried out to recover shale oil from the oil shale.
U.S. patent application Ser. No. 070,319 discloses a method for explosively expanding formation containing oil shale towards a horizontal free face to form a fragmented mass in an in situ oil shale retort. According to such a method, a void having a horizontal cross-section similar to the horizontal cross-section of the retort being formed is initially excavated. A plurality of vertically spaced apart zones of unfragmented formation are left above the void. Explosive is placed in each of the unfragmented zones and detonated for explosively expanding such zones towards the void to form a fragmented mass in the retort having an average void volume about equal to the void volume of the initial void. The overlying zones can be expanded towards the void in a single round or a plurality of rounds. Retorting of the fragmented mass is then carried out to recover shale oil from the oil shale.
It is desirable to have a generally uniformly distributed void fraction in the fragmented mass so that there is generally uniform permeability. Thus, oxygen-supplying gas and combustion gas can flow reasonably uniformly through the fragmented mass during retorting operations. A fragmented mass having generally uniform permeability avoids bypassing portions of the fragmented mass by retorting gas as can occur if there is gas channeling through the mass due to non-uniform permeability.
When using vertical columnar explosive charges for explosively expanding formation, some of the blastholes containing such charges can be located close to the vertical walls of a void towards which expansion is directed. The charges in these blastholes are not free to crater toward the horizontal free face (i.e., upward or downward, as the case may be), but are confined on one side by the wall. Formation expanded by these charges is directed in some measure inwardly away from the walls and not entirely vertically, as desired. This can cause the fragmented mass of formation particles formed to have a higher void fraction along the walls of the retort and a lower void fraction near the center. Having a fragmented permeable mass with a higher void fraction along the walls and a lower void fraction in about the center can result in gas channeling along the walls and consequent reduction in the efficiency of the retorting process.
In the past, columnar explosive charges in the blastholes adjacent the plane of the vertical walls of a void have been provided which are about the same size or energy and have about the same spacing distance as explosive charges remote from the side walls.
The "spacing distance" as used herein is the distance between adjacent explosive charges or blastholes.
Explosive in a blasthole adjacent the plane of a vertical wall of a void can expand only that portion of the zone of unfragmented formation which can break to an adjacent free face. Therefore, an explosive charge adjacent a vertical wall is free to expand only about one-half the volume of formation that it could expand if it were located remote from the wall. As a consequence, more explosive has been provided in charges along such side walls than is actually required to expand the unfragmented formation in this region.
When charges adjacent a side wall are more energetic than necessary, formation is expanded to a greater extent than necessary toward the center of the retort. This enhances the uneven distribution of void fraction across the horizontal extent of the fragmented permeable mass of formation particles formed in the retort. Also, using more explosive than necessary results in an increased expense, both because of the additional explosive used and also because of the added expense of drilling larger blastholes than required along the walls.
Additionally, when the amount of explosive used to expand a given volume of formation in a region adjacent a void wall is greater than the amount of explosive used to expand a given volume of formation more remote from the void wall, fragmentation of formation is not uniform. That is, particles formed by detonation of the explosive charge adjacent the side wall are smaller than particles formed by detonation of charges more remote from the side wall.
It is, therefore, desirable to provide an economical method for expanding formation toward a horizontal free face which results in a fragmented mass of formation particles having a reasonably uniform particle size distribution and void fraction distribution.